Radially embedded permanent magnet rotor and methods thereof

ABSTRACT

In one embodiment, a permanent magnet rotor for use with a stator is provided. The permanent magnet rotor includes at least one permanent magnet and a substantially cylindrical rotor core including an outer edge, a hub having an inner edge defining a central opening, and a plurality of independent pole pieces spaced from the hub radially about the central opening. The rotor core further includes at least one radial aperture extending radially from the outer edge through the rotor core between adjacent independent pole pieces, the at least one aperture configured to receive the at least one permanent magnet.

BACKGROUND

The field of the disclosure relates generally to electric motors, andmore particularly, to radially embedded permanent magnet rotors andalternative materials for use in electric motors.

Various types of electric machines include permanent magnets. Forexample, a brushless direct current (BLDC) motor may include a pluralityof permanent magnets coupled to an exterior surface of a rotor core.Typically, the permanent magnets are coupled to the exterior surface ofthe rotor core using an adhesive and/or an outer retaining covering.This coupling between the permanent magnets and the rotor core mustresist forces exerted on the permanent magnets during high speedrotation tending to separate the permanent magnets from the motor.

Permanent magnets may also be positioned within a rotor core, commonlyreferred to as an interior permanent magnet rotor. Slots are formedwithin the rotor, and magnets are inserted into the slots. The magnetslots must be larger than the magnets to allow the magnets to beinserted. However, the magnets must be secured within the slots toprevent movement of the magnets during operation of the machine. Theperformance of the machine depends on maintaining the magnets in a knownposition within the rotor. An adhesive may be used to secure the magnetsin a fixed position relative to the rotor. However, adhesives have alimited life due to factors such as temperature, temperature cycling,and environmental conditions.

Many known electric machines produce work by generating torque, which isthe product of flux, stator current and other constants. In electricmotors, flux is typically produced by permanent magnets positioned on arotor within the motor. Some known rare earth permanent magnets, such asneodymium iron boron magnets, generate greater amounts of flux thantypical ferrite permanent magnets. However, the cost of rare earthmagnets has drastically risen in recent years, prompting the need forlow-cost permanent magnet systems that generate similar amounts of fluxand provide efficiencies similar to systems using rare earth magnets.

BRIEF DESCRIPTION

In one embodiment, a permanent magnet rotor for use with a stator isprovided. The permanent magnet rotor includes at least one permanentmagnet and a substantially cylindrical rotor core including an outeredge, a hub having an inner edge defining a central opening, and aplurality of independent pole pieces spaced from the hub radially aboutthe central opening. The rotor core further includes at least one radialaperture extending radially from the outer edge through the rotor corebetween adjacent independent pole pieces, the at least one apertureconfigured to receive the at least one permanent magnet.

In another embodiment, a method of manufacturing a permanent magnetrotor for use with a stator is provided. The method includes providing asubstantially cylindrical rotor core including an outer edge, a hubhaving an inner edge defining a central opening, and a plurality ofindependent pole pieces spaced from the hub radially about the centralopening. The method further includes positioning the plurality ofindependent pole pieces to form at least one radial aperture extendingradially from the outer edge through the rotor core between adjacentindependent pole pieces and inserting a permanent magnet into the atleast one radial aperture.

In yet another embodiment, a permanent magnet rotor for use with astator is provided. The rotor includes at least one permanent magnet anda substantially cylindrical rotor core including an outer edge, a hubhaving an inner edge defining a central opening, and a plurality ofindependent pole pieces spaced from the hub radially about the centralopening. The rotor further includes at least one radial apertureincluding opposed first and second edges, the radial aperture configuredto receive the at least one permanent magnet and extending radially fromthe outer edge through the rotor core between adjacent independent polepieces, wherein a distance between the first and second opposed edges isgreater than three multiples of an air gap distance between the rotorouter edge and an inner edge of the stator. The rotor includes at leastone shorting lamination coupled to the rotor core, wherein a thicknessof the at least one shorting lamination is less than 12% of the totallength of the rotor core

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective cut-away view of an exemplary electric machine;

FIG. 2 is a front view of an exemplary rotor core that may be includedwithin the electric machine shown in FIG. 1;

FIG. 3 is a front view of another exemplary rotor core that may beincluded within the electric machine shown in FIG. 1;

FIG. 4 is a front view of another exemplary rotor core that may beincluded within the electric machine shown in FIG. 1;

FIG. 5 is a front view of the exemplary rotor core shown in FIG. 3positioned within a stator core;

FIG. 6 is an exploded view of another exemplary rotor core that may beincluded within the electric machine shown in FIG. 1;

FIG. 7 is a front view of another exemplary rotor core that may beincluded within the electric machine shown in FIG. 1;

FIG. 8 is an expanded view of the rotor core shown in FIG. 7 with aretention material therein;

FIG. 9 is an expanded view of the rotor core shown in FIG. 7 withanother retention material therein;

FIG. 10 is a perspective view of another exemplary rotor core that maybe included within the electric machine shown in FIG. 1;

FIG. 11 is a perspective view of the rotor core of FIG. 10 with arotated end lamination and permanent magnets;

FIG. 12 is a front view of an exemplary lamination that may be includedwith the rotor core shown in FIG. 10;

FIG. 13 is a front view of another exemplary lamination that may beincluded with the rotor core shown in FIG. 10;

FIG. 14 is a front view of another exemplary lamination that may beincluded with the rotor core shown in FIG. 10;

FIG. 15 is a front view of another exemplary rotor core that may beincluded within the electric machine shown in FIG. 1;

FIG. 16 is a perspective sectional view of another exemplary rotor corethat may be included within the electric machine shown in FIG. 1;

FIG. 17 is a perspective view of another exemplary rotor core that maybe included within the electric machine shown in FIG. 1;

FIG. 18 is a perspective view of the rotor core shown in FIG. 17 withwebs mechanically disconnected;

FIG. 19 is a perspective view of another exemplary rotor core that maybe included within the electric machine shown in FIG. 1;

FIG. 20 is a perspective cut-away view of another exemplary electricmachine;

FIG. 21 is a chart plotting motor speed and torque values; and

FIG. 22 is a perspective cut-away view of another exemplary electricmachine.

FIG. 23 is a schematic view of an exemplary fluid moving system.

DETAILED DESCRIPTION

Due to increased costs of rare earth magnets and copper used forwindings, lower cost alternative materials are desirable in the designand manufacture of electric motors. This disclosure provides designs andmethods using material alternatives to rare earth magnets and copperwindings while reducing or recapturing the efficiency losses associatedwith those alternative materials and reducing or eliminating an increaseof the length of the motor.

FIG. 1 is a perspective cut-away view of an exemplary electric motor 10.Although referred to herein as electric motor 10, electric motor 10 canbe operated as either a generator or a motor. Electric motor 10 includesa first end 12, a second end 14, and a motor assembly housing 16.Electric motor 10 also includes a stationary assembly 18 and a rotatableassembly 20. Motor assembly housing 16 defines an interior 22 and anexterior 24 of motor 10 and is configured to at least partially encloseand protect stationary assembly 18 and rotatable assembly 20. Stationaryassembly includes a stator core 28, which includes a plurality of teeth30 and a plurality of windings 32 wound around stator teeth 30. In theexemplary embodiment, stator core 28 is a twelve tooth stator structure.Alternatively, stator core 28 may include any number of teeth thatenables motor 10 to function as described herein, for example, statorcore 28 may have nine teeth. Furthermore, in an exemplary embodiment,stationary assembly 18 is a three-phase salient pole stator assembly andstator core 28 is formed from a stack of laminations made of highlymagnetically permeable material. Alternatively, stationary assembly 18is a single phase salient pole stator assembly. Stationary assembly 18may be a round, segmented, or roll-up type stator construction andwindings 32 are wound on stator core 28 in any suitable manner thatenables motor 10 to function as described herein. For example, windings32 may be concentrated type or overlapped type windings.

Rotatable assembly 20 includes a permanent magnet rotor core 36 and ashaft 38. In the exemplary embodiment, rotor core 36 is formed from astack of laminations made of magnetically permeable material.Alternatively, rotor core 36 is a solid core. Rotor core 36 issubstantially received in a central bore of stator core 28 for rotationalong an axis of rotation X. FIG. 1 illustrates rotor core 36 and statorcore 28 as solid for simplicity. While FIG. 1 is an illustration of athree phase electric motor, the methods and apparatus described hereinmay be included within motors having any number of phases, includingsingle phase and multiple phase electric motors.

In the exemplary embodiment, electric motor 10 is coupled to a fan orcentrifugal blower, as shown in FIG. 23, for moving air through an airhandling system, for blowing air over cooling coils, and/or for drivinga compressor within an air conditioning/refrigeration system. Morespecifically, motor 10 may be used in air moving applications used inthe heating, ventilation, and air conditioning (HVAC) industry, forexample, in residential applications using ⅕ horsepower (hp) to 1 hpmotors. Alternatively, motor 10 may be used in fluid pumpingapplications. Motor 10 may also be used in commercial and industrialapplications,and/or hermetic compressor motors used in air conditioningapplications, where motor 10 may have a rating of greater than 1 hp.Although described herein in the context of an air handling system,electric motor 10 may engage any suitable work component and beconfigured to drive such a work component.

FIG. 2 is a front view of an exemplary embodiment of rotor core 36 thatmay be included within electric motor 10 (shown in FIG. 1). In theexemplary embodiment, rotatable assembly 20, also referred to as aradially embedded permanent magnet rotor, includes a rotor core 36 and ashaft 38. Examples of motors that may include the radially embeddedpermanent magnet rotors include, but are not limited to, electronicallycommutated motors (ECM's). ECM's may include, but are not limited to,brushless direct current (BLDC) motors, brushless alternating current(BLAC) motors, and variable reluctance motors. Furthermore, rotatableassembly 20 is driven by an electronic control (not shown), for example,a sinusoidal or trapezoidal electronic control.

Rotor core 36 is substantially cylindrical and includes an outer edge 40and a shaft central opening or inner edge 42 having a diametercorresponding to the diameter of shaft 38. Rotor core 36 and shaft 38are concentric and are configured to rotate about axis of rotation X(shown in FIG. 1). In the exemplary embodiment, rotor core 36 includes aplurality of laminations 44 that are either interlocked or loose. Forexample, laminations 44 are fabricated from multiple punched layers ofstamped metal such as steel. In an alternative embodiment, rotor core 36is a solid core. For example, rotor core 36 may be fabricated using asintering process from a soft magnetic composite (SMC) material, a softmagnetic alloy (SMA) material, and/or a powdered ferrite material.

In the exemplary embodiment, rotor core 36 includes a plurality ofradial apertures 46. For example, a first wall 48, a second wall 50 anda third wall 52 define a first radial aperture 54 of the plurality ofradial apertures 46. Each radial aperture 46 includes a depth d andthickness t and extends axially through rotor core 36 from first end 12(shown in FIG. 1) to second end 14 (also shown in FIG. 1). Each radialaperture 46 is configured to receive one or more permanent magnets 56such that each magnet 56 is radially embedded in rotor core 36 andextends at least partially from rotor first end 12 to rotor second end14. In the exemplary embodiment, permanent magnets 56 are hard ferritemagnets magnetized in a direction tangent to axis of rotation X.However, magnet 56 may be fabricated from any suitable material thatenables motor 10 to function as described herein, for example, bondedneodymium, sintered neodymium, and/or samarium cobalt.

In the exemplary embodiment, rotor core 36 includes a plurality of rotorpoles 58, each having an outer wall 60 along rotor outer edge 40 and aninner wall 62 (shown in FIG. 3). In the exemplary embodiment, the numberof radial apertures 46 is equal to the number of rotor poles 58, and onemagnet 56 is positioned within each radial aperture 46 between a pair ofrotor poles 58. Although illustrated as including ten rotor poles 58,rotor core 36 may have any number of poles that allows motor 10 tofunction as described herein, for example, six, eight or twelve poles.

In the exemplary embodiment, the design of radially embedded permanentmagnet rotor core 36 utilizes lower-cost magnets, yet achieves the powerdensities and high efficiency of machines using higher-cost magnets,such as neodymium magnets. In the exemplary embodiment, increasedefficiency and power density of motor 10 is obtained by increasing theflux produced by rotor core 36. Increased flux generation is facilitatedby magnets 56 positioned in radial apertures 46 at depth d, between aminimum magnet depth and a maximum magnet depth. The minimum magnetdepth is defined by the equation:

${D_{\min} = \frac{\left( {\pi*R} \right)}{n}},$wherein D_(min) represents the minimum depth variable, R represents therotor radius, and n represents the number of rotor poles. The maximummagnet depth is defined by the equation:

${D_{\max} = {R - \frac{0.5t}{\tan\left( \frac{180}{n} \right)}}},$wherein D_(max) represents the maximum depth variable, R represents therotor radius, t represents the magnet thickness in the direction ofmagnetization, and n represents the number of rotor poles. In theexemplary embodiment, rotor core 36 facilitates increased fluxproduction resulting in optimum efficiency and power density whenmagnets 56 extend into radial aperture at a depth between D_(min) andD_(max).

FIG. 3 is a front view of another exemplary embodiment of rotor core 36that may be included within electric motor 10. In the exemplaryembodiment, rotor core 36 includes radial apertures 46 configured toreceive one or more permanent magnets 56. In the exemplary embodiment,radial apertures 46 are generally rectangular. Alternatively, radialapertures 46 may have any suitable shape corresponding to the shape ofthe permanent magnets that enables electric motor to function asdescribed herein. For example, radial apertures 46 may be tapered, asdescribed in more detail below.

In the exemplary embodiment, radial aperture 46 includes one or morepermanent magnet retention member or protrusion 64. For example, a firstpair of protrusions 66 is located proximate pole outer wall 60 alongrotor outer edge 40 and extends into radial aperture 46 from first andsecond walls 48 and 50. Each protrusion 64 of the first pair ofprotrusions 66 is configured to facilitate retention of magnet 56 withinradial aperture 46 by substantially preventing movement of magnet 56 ina radial direction towards outer edge 40. Further, a second pair ofprotrusions 68 is located along pole inner wall 62 and extends intoradial aperture 46 from first and second walls 48 and 50. Eachprotrusion 64 of the second pair of protrusions 68 is configured tofacilitate retention of magnet 56 within radial aperture 46 bysubstantially preventing movement of magnet 56 in a radial directiontowards shaft 38. Alternatively, rotor core 36 may have any number andlocation of protrusions 64 that enable rotor core 36 to function asdescribed herein.

FIG. 4 is a front view of another exemplary embodiment of rotor core 36that may be included within electric motor 10. Rotor core 36 includesradial apertures 46 configured to receive one or more permanent magnet56. In the exemplary embodiment, radial apertures 46 and magnet 56 aregenerally tapered. First and second walls 48 and 50 of radial aperture46 converge as they extend from rotor inner wall 62 to rotor outer wall60 and are configured to engage the tapered walls of magnet 56 tofacilitate retention of magnet 56 within radial aperture 46 bysubstantially preventing movement of magnet 56 in a radial directiontowards rotor outer edge 40. Furthermore, each tapered radial aperture46 may include a pair of protrusions 68 located along pole inner wall 62to facilitate retention of magnet 56 within radial aperture 46 bysubstantially preventing movement of magnet 56 in a radial directiontowards shaft 38.

FIG. 5 is a front view of rotor core 36 shown in FIG. 3 positionedwithin stator core 28. In the exemplary embodiment, rotor core 36 ispositioned relative to stator core 28, and rotor outer edge 40 and aninner edge 74 of stator core 28 define a small air gap 72. Air gap 72allows for relatively free rotation of rotor core 36 within stator core28. Radially embedded magnets 56 of rotor core 36 are configured tofacilitate increased flux to air gap 72, resulting in increased motortorque generation. The radial orientation of radially embedded magnets56 results in the magnet flux only crossing the magnet once, as opposedto the flux produced by surface-mounted magnets, which must cross themagnets twice. Crossing magnet 56 only once significantly reduces thepath of the flux through material of low permeability (i.e. air andmagnet 56), resulting in increased flux delivery and torque. Increasedflux delivery and torque also result from radial magnets 56 of the samepolarity positioned on opposite edges 48 and 50 of each rotor pole 58,which focuses flux toward rotor outer edge 40. However, any magneticsupport structure above or below magnet 56 in a radial directionprovides a path for flux to flow without crossing air gap 72, resultingin torque losses. In the exemplary embodiment of FIG. 5, only a small orlimited amount of magnetic material (i.e. protrusions 64) is positionedabove or below magnet 56. Alternatively, rotor core 36 does not includeany magnetic material immediately above or below magnet 56 such that nomagnetic material is positioned between permanent magnet 56 and rotorouter edge 40 and between permanent magnet 56 and inner sleeve 138 (seeFIG. 15).

In the exemplary embodiment, rotor poles 58 are spaced from each other adistance f to reduce flux loss through magnetic support structure (e.g.rotor poles 58). In the exemplary embodiment, distance f is greater thanor equal to five times the length of air gap 72 (the gap between rotorouter edge 40 and stator inner edge 74), facilitating high fluxgeneration. Alternatively, distance f is greater than or equal to threetimes the length of air gap 72. Alternatively still, distance f isgreater than or equal to ten times the length of air gap 72. In theexemplary embodiment, distance f is maintained between protrusions 64.Alternatively, distance f is maintained between radial aperture walls 48and 50 if no protrusions 64 are present, or between protrusion 64 andwall 48 or 50 if protrusion 64 is present on only one of walls 48 and50.

FIG. 6 is an exploded view of another exemplary embodiment of rotor core36 that may be included within electric motor 10. In the exemplaryembodiment, rotor core 36 includes a first half-core 76, a secondhalf-core 78, a center lamination 80, and first and second endlaminations 82 and 84. Half-cores 76 and 78 each include a plurality ofindependent rotor poles 58 positioned radially about a sleeve 86. Aplurality of radial apertures 46 are defined between rotor poles 58 andare configured to receive one or more permanent magnets 56. Each rotorpole 58 is held in spaced relation to sleeve 86 by at least one ofcenter lamination 80 and end laminations 82 and 84. In the exemplaryembodiment, laminations 80, 82 and 84, also referred to as shortinglaminations, are structurally similar, and each includes a plurality ofconnected rotor poles 88 positioned radially about a central hub 90.Rotor poles 88 each include an outer edge 92 and an inner edge 94.Adjacent pairs 96 of rotor poles 88 are connected at inner edges 94 by abridge 98, which is connected to central hub 90.

In the exemplary embodiment, center lamination 80 is positioned betweenhalf-cores 76 and 78, and end laminations 82 and 84 are positioned onopposite ends of rotor core 36. In the exemplary embodiment, half-cores76 and 78 are solid cores. Alternatively, half-cores 76 and 78 areformed as a whole core and/or are fabricated from a plurality oflamination layers. Although rotor core 36 is described with a singlecenter lamination 80 and two end laminations 82 and 84, rotor core 36may have any number of center and end laminations that enables motor 10to function as described herein. Connected rotor poles 88 support rotorpoles 58 at a distance from sleeve 86 to prevent flux losses inhalf-cores 76 and 78, since little or no magnetic material is locatedabove or below magnets 56 positioned therein. A portion of fluxgenerated by rotor core 36 is lost, however, due at least in part toconnected rotor poles 88 of laminations 80, 82 and 84. In order tominimize flux losses, in the exemplary embodiment, the sum of thethicknesses of laminations having connected rotor poles 88 is less thanor equal to 12% of the total length of rotor core 36. Alternatively, thesum of the thicknesses of laminations having connected rotor poles 88 isless than or equal to 2% of the total length of rotor core 36.Alternatively still, the sum of the thicknesses of laminations havingconnected rotor poles 88 is less than or equal to 1% of the total lengthof rotor core 36.

FIGS. 7-9 illustrate another exemplary embodiment of rotor core 36 thatmay be included within electric motor 10. FIG. 7 is a front view ofrotor core 36 that includes a hub 100 defining an inner edge 42 sized toreceive shaft 38, and a plurality of rotor poles 58 positioned radiallyabout hub 100 defining a plurality of radial apertures 46, which areconfigured to receive one or more permanent magnet 56. In the exemplaryembodiment, hub 100 is fabricated from aluminum and/or zinc and presseddirectly onto shaft 38. Alternatively, hub 100 is fabricated from anynon-magnetic material that enables rotor core 36 to function asdescribed herein. In the exemplary embodiment, a first indentation 102is formed in each of first wall 48 and second wall 50, and two secondindentations 104 are formed in magnet 56. Alternatively, only one ofindentation 102 and indentation 104 is formed, or any number andlocation of indentations 102 and 104 may be formed in walls 48 and 50and magnet 56 that enables rotor 36 to function as described herein.Moreover, indentations 102 and 104 may have any suitable shape and sizethat enables rotor core 36 to function as described herein. For example,second indentation 104 may be smaller in size and/or have a smallerradius than first indentation 102.

In the exemplary embodiment, each first indentation 102 is substantiallyaligned with a corresponding second indentation 104 to define a space106. Alternatively, first indentations 102 are formed without formingsecond indentations 104, and vice versa. Space 106 is configured toreceive a retention material 108, which is configured to at leastpartially fill space 106 and cause interference between the surfaces ofindentations 102 and 104 to substantially resist or prevent movement ofmagnet 56 within radial aperture 46. For example, retention material 108frictionally engages the surfaces of indentations 102 and 104 andprevents magnet 56 from moving radially relative to radial aperture 46,which can result in unwanted noise or magnet dislocation. Further,retention material 108 is configured to prevent general side-to-sidemotion of magnet 56, which can occur due to tolerance stack-up and canresult in unwanted noise from magnets 56 rattling against the faces ofrotor poles 58. Retention material 108 may be formed from one segment ormultiple segments and extend through the full length or through only aportion of rotor core 36.

In the exemplary embodiment, retention material 108 may be any materialor member that at least partially fills space 106 and substantiallyprevents movement of magnet 56 within radial aperture 46. For example,as illustrated in FIG. 8, the retention material is a resilient hollowouter member 110 positioned within space 106. However, retentionmaterial 108 is not limited to a single material. For example, ahardened material member (not shown) may be inserted through hollowouter member 110 to facilitate additional interference betweenindentations 102 and 104. An alternative embodiment is shown in FIG. 9,which illustrates an expanding foam 112 as the retention material.Expanding foam 112 is deposited within space 106, and expands to engagethe surfaces of indentations 102 and 104 and facilitate retention ofmagnet 56 within radial aperture 46. Alternatively, retention material108 may be, for example, a pre-fabricated plastic or metal member (notshown), a formed in place member (not shown) such as with injectionmolded polymer, plastic or metal or an expanding material, a resilientmember (not shown), an elastic member (not shown), a wedging member (notshown), a biasing mechanism such as a spring (not shown), or anycombination thereof. Furthermore, retention material 108 may be used inplace of, or in addition to, adhesives to retain magnet 56 within radialaperture 46.

FIG. 10 illustrates a perspective view of another exemplary embodimentof rotor core 36 that may be included within electric motor 10. In theexemplary embodiment, rotor core 36 is formed from a plurality oflaminations 44 that each includes a hub 114, a plurality of connectedrotor poles 116, and a plurality of independent rotor poles 118. Rotorpoles 116 and 118 are positioned radially about hub 114. Each connectedrotor pole 116 is coupled to hub 114 by a web 120. In the exemplaryembodiment, a sleeve 122 fabricated from a non-magnetic material, forexample aluminum, is positioned between hub 114 and shaft 38 to providemechanical stability and prevent a magnetic path to shaft 38.Alternatively, sleeve 122 is excluded and/or shaft 38 is at leastpartially formed from a non-magnetic material to prevent a magnetic paththereto. Radial apertures 46 are defined between adjacent rotor poles116 and 118 and are configured to receive one or more permanent magnet56. Rotor poles 116 and 118 also include magnet retention features 64configured to retain magnet 56 within radial aperture 46, as describedabove. Illustrated in FIG. 11, one or more laminations 124 are rotatedto support independent rotor poles 118 in spaced relation to hub 114.Each rotated lamination 124 includes a plurality of connected rotorpoles 126 and independent rotor poles 128. Rotated lamination 124 isoriented or rotated in a clockwise or counterclockwise direction by onepole such that each connected rotor pole 126 is aligned with andsupports one independent rotor pole 118, and each independent rotor pole128 is aligned and supported by one connected rotor pole 116.

In the exemplary embodiment, two rotated laminations 124 are positionedon rotor first end 12. Additionally, one or more rotated laminations 124are positioned on rotor second end 14. However, any number of rotatedlaminations 124 may be positioned anywhere throughout the stack oflaminations 44 that enables rotor core 36 to function as describedherein. For example, rotated lamination 124 may be located substantiallywithin the center of the lamination stack of rotor core 36. In theexemplary embodiment, rotated lamination 124 is fabricated from steel.Alternatively, rotated lamination 124 is formed from any non-magneticmaterial such as aluminum or molded plastic.

In the exemplary embodiment, each permanent magnet 56 is positionedwithin one radial aperture 46 by inserting magnet 56 therethrough in anaxial direction parallel to axis X. Alternatively, each permanent magnet56 is inserted into one radial aperture 46 in a direction radial to hub114, for example, when protrusions 64 are not present on outer edge 40.Each permanent magnet 56 includes a first polarity pole N and a secondpolarity pole S. In the exemplary embodiment, magnets 56 are arrangedwithin radial aperture 46 such that each pole N faces one connectedrotor pole 116 and each pole S faces one independent rotor pole 118.This arrangement results in little or no flux leakage because each rotorpole 116 and 118 is only in contact with the same magnetic polarity.Alternatively, magnets 56 are arranged such that each pole S faces oneconnected rotor pole 116 and each pole N faces one independent rotorpole 118.

FIGS. 12-14 illustrate alternate exemplary embodiments of laminations 44positioned on rotor ends 12 and 14 or anywhere throughout laminatedrotor core 36. For example, FIG. 12 illustrates a front view of alamination 130 that is similar to lamination 124 illustrated in FIG. 10,except lamination 130 includes apertures 132 and indentations 102 formedin connected rotor poles 116 and independent rotor poles 118.Indentations 102 are configured to align with indentations 104 formed inmagnets 56 to define a space 106, which is configured to receive aretention material 108, as described above. Apertures 132 are configuredto receive fasteners (not shown) to facilitate aligning and couplingmultiple laminations 130. FIG. 13 illustrates a front view of alamination 134 that is similar to lamination 130, except connected rotorpoles 116 and independent rotor poles 118 are connected by bridges 136.FIG. 14 illustrates a front view of a lamination 137 that is similar tolamination 134, except lamination 137 excludes indentations 102 andapertures 132. Moreover, when used as end laminations, the laminationsdescribed above may include radial apertures 46 sized smaller thanmagnets 56 to prevent magnet 56 from sliding out a rotor end duringaxial insertion of magnet 56.

FIG. 15 is a front view of another exemplary embodiment of rotor core 36that may be included within electric motor 10. In the exemplaryembodiment, rotor core 36 includes a sleeve 138 configured to receiveshaft 38, a central hub 140, and a plurality of rotor poles 58positioned radially about central hub 140. Rotor poles 58 are solid orlaminated and define a plurality of radial apertures 46 therebetween,which are configured to receive one or more permanent magnet 56. Rotorpoles 58 include indentations 102 that are substantially aligned withindentations 104 in permanent magnets 56 to form a space 106. Each rotorpole 58 includes a protrusion 142 that extends into a recess 144 formedin central hub 140. Each protrusion 142 and corresponding recess 144forms an interlock 146. In the exemplary embodiment, interlock 146 is adovetail joint configured to facilitate increased torque transmissionbetween rotor poles 58 and central hub 140. Alternatively, interlock 146may have any geometry that enables rotor core 36 to function asdescribed herein.

In the exemplary embodiment, radial aperture 46 includes first andsecond walls 48 and 50 defined by rotor poles 58. A retention material108 is positioned within radial aperture 46 and space 106 between magnet56 and first and second walls 48 and 50. An annular end plate (notshown) is positioned on rotor core ends. In the exemplary embodiment,retention material 108 is an injection molded polymer. However,retention material 108 may be any suitable material that enables rotorcore 36 to function as described herein. Additionally, retentionmaterial 108 may be positioned between magnet 56 and an outer edge 40 ofrotor core 36. In the exemplary embodiment, sleeve 138 and rotor poles58 are fabricated from steel. However, sleeve 138 and rotor poles 58 maybe formed from any suitable material that enables rotor core 36 tofunction as described herein. Alternatively, sleeve 138 may be excludedand central hub 140 is directly coupled to shaft 38. In the exemplaryembodiment, central hub 140 is fabricated from an injection moldedpolymer. However, central hub 140 may be formed from any suitablenon-magnetic material that enables rotor core 36 to function asdescribed herein. For example, central hub 140 may be a machined,extruded or die cast non-magnetic material such as aluminum or zinc.Alternatively, central hub 140 is fabricated from an isolation dampingmaterial configured to reduce transmission of at least one of motortorque pulsations, motor torque ripple, and motor torque cogging.

An exemplary method of manufacturing rotor core 36 is described herein.Sleeve 138, rotor poles 58 and permanent magnets 56 are locatedsubstantially symmetrically in a mold (not shown). Alternatively, sleeve138 may be excluded. Rotor poles 58 and sleeve 138 are a solid structureor laminated structure held together with interlocks, rivets, bolts,and/or other fasteners. In the exemplary embodiment, pre-formed magnets56 are inserted into radial apertures 46 defined between rotor poles 58.Alternatively, a polymer bonded magnet material is injected into theradial apertures 46 to form permanent magnets 56. In the exemplaryembodiment, a non-magnetic polymer is injection molded into the regionbetween rotor poles 58 and magnets 56, in the region between magnets 56and rotor outer edge 40, and in spaces 106 defined by indentations 102and 104. The polymer is further injection molded in the region betweensleeve 138 and rotor poles 58 and magnets 56 to form central hub 140. Inthis way, the injection molding process magnetically isolates rotorpoles 58 and magnets 56 on the outer diameter of rotor 36 and orientsrotor poles 58 and magnets 56 at a predetermined diameter equal to adesired rotor outer diameter. Magnets 56 may be magnetized beforeinsertion into the mold or may be magnetized after the molding process.Alternatively, central hub 140 is a pre-formed non-magnetic material,for example zinc or aluminum, that is inserted into the mold aftermagnets 56 are installed and magnetized. An annular end plate (notshown) is then positioned on rotor core ends. Alternatively, sleeve 138and rotor poles 58 are located substantially symmetrically in the mold.A high strength polymer material with a high first processingtemperature (e.g. glass-filled Rynite material) is injection molded inthe region between sleeve 138 and rotor poles 58 to form central hub140. The processing temperature is the temperature at which a materialcan be processed in a conventional injection molding process and isabove the higher of the melting point and the glass transitiontemperature. Permanent magnets 56 are then inserted into radialapertures 46 defined between rotor poles 58, and a polymer material witha lower second processing temperature (e.g. glass-filled Crastin) ismolded in the remaining space around magnets 56. This two-step moldingprocess prevents performing the high temperature material injectionwhile magnets 56 are in the mold, which may reduce the effectiveness ofmagnets 56.

FIG. 16 is a section view of another exemplary embodiment of rotor core36 that may be included within electric motor 10. In the exemplaryembodiment, rotor core 36 includes a sleeve 138 configured to receiveshaft 38, a central hub 140, and a plurality of rotor poles 58positioned radially about central hub 140. Sleeve 138 includesindentations 150 to provide an interlock with central hub 140 tofacilitate increased torque transmission therebetween. In the exemplaryembodiment, hub 140 is solid. Alternatively, hub 140 is fabricated withapertures 152 (see FIG. 17). Rotor poles 58 are a solid or laminatedstructure and define a plurality of radial apertures 46, which areconfigured to receive one or more permanent magnets 56. Rotor poles 58include protrusions 64, which are configured to retain magnets 56 withinradial apertures 46, as described above. Each rotor pole 58 includes arecess 144 that receives protrusion 142 extending from central hub 140.Each protrusion 142 and corresponding recess 144 forms an interlock 146,which is configured to facilitate increased torque transmission betweenrotor poles 58 and central hub 140. In an alternate embodiment,interlock 146 may have any geometry that enables rotor core 36 tofunction as described herein. Alternatively, rotor poles 58 are coupledto central hub 140 in any manner that enables torque transmissionbetween rotor poles 58 and central hub 140, for example, by adhesives,mechanical fasteners, etc.

In the exemplary embodiment, radial aperture 46 receives one or moremagnet 56 and a retention material 108 is positioned within radialaperture 46 between magnet 56 and rotor outer edge 40. As illustrated inFIG. 17, one or more end laminations 154 are positioned on rotor ends 12and 14. In the exemplary embodiment, each end lamination 154 includes ahub 156 connected to a plurality of rotor poles 158 by webs 160.Alternatively, since webs 160 provide a magnetic path through the rotorand between rotor poles 158, which results in some flux leakage, webs160 may be mechanically disconnected or sheared from hub 156 (see FIG.18) and then molded over when forming hub 156. Mechanicallydisconnecting webs 160 provides resiliency between the rotor outerdiameter and hub 140, which reduces torque pulsations, vibration, noiseand undesirable cogging torque and/or ripple torque. Alternatively, oneor more bridges (not shown) are formed between adjacent rotor poles 158along rotor outer diameter 40 and/or anywhere along radial apertures 46.As such, the bridges of end lamination 154 secure rotor core 36 andincrease rigidity during punch and interlock, stacking, and/ortransport. Once rotor core 36 is formed, the bridges may be mechanicallydisconnected for plastic molding, potting and/or epoxy fillingoperations.

An exemplary method of manufacturing rotor core 36 is described herein.Rotor core 36 is fabricated by punching multiple interlocked laminationsthat each includes a sleeve 138 and a plurality of rotor poles 58. Thelaminations are stacked and may or may not be indexed to reduce rotorimbalances. One or more magnets 56 are positioned radially about sleeve138 between rotor poles 58 by inserting each magnet 56 axially and/orradially into radial aperture 46. One or more end laminations 154 arepositioned on rotor core end 12 and/or 14, and rotor core 36 is locatedin a mold (not shown). Optionally, a plurality of rotor poles 158 of endlamination 154 may be mechanically disconnected from a hub 156 bybreaking or shearing webs 160. A non-magnetic polymer is injectionmolded into the region between rotor poles 58 and magnets 56, in theregion between magnets 56 and rotor outer edge 40, and in the regionbetween sleeve 138 and rotor poles 58 and magnets 56 to form central hub140. The polymer may also be injection molded over mechanicallydisconnected webs 160. In this way, the injection molding processmagnetically isolates rotor poles 58 and magnets 56 on the outerdiameter of rotor 36 and orients rotor poles 58 and magnets 56 at apredetermined diameter equal to a desired rotor outer diameter. Magnets56 may be magnetized before insertion into the mold, or may bemagnetized during or after the molding process. End lamination 154 maybe coupled to rotor core 36 before, during or after the molding process.

FIG. 19 is a section view of another exemplary embodiment of rotor core36 that may be included within electric motor 10. In the exemplaryembodiment, rotor core 36 includes an inner hub 162 configured toreceive shaft 38, an outer hub 164, and a plurality of rotor poles 58positioned radially about outer hub 164. Rotor poles 58 are solid orlaminated and define a plurality of radial apertures 46 therebetween,which are configured to receive one or more permanent magnet 56. Eachrotor pole 58 includes a protrusion 142 that extends into a recess 144in outer hub 164 to form an interlock 146. In the exemplary embodiment,interlock 146 is a dovetail joint configured to facilitate increasedtorque transmission between rotor poles 58 and outer hub 164.

In the exemplary embodiment, radial aperture 46 receives one or morepermanent magnet 56. A molded material (e.g. injection molded polymer)may be positioned within radial aperture 46 between magnet 56 and rotorouter edge 40 and between magnet 56 and outer hub 164. Similarly, thematerial is positioned in a space 166 between outer hub 164 and innerhub 162 to form a central hub (not shown). Inner and outer hubs 162 and164 each include protrusions 168, which facilitate increased torquetransmission therebetween. In the exemplary embodiment, inner and outerhubs 162 and 164 are fabricated from a non-magnetic material (e.g.aluminum). Alternatively, hubs 162 and 164 are fabricated from anynon-magnetic material that enables rotor 36 to function as describedherein. Alternatively, inner hub 162 is fabricated from a magneticmaterial.

An exemplary method of manufacturing rotor core 36 is described herein.Inner hub 162, outer hub 164, rotor poles 58 and magnets 56 are locatedsubstantially symmetrically in a mold (not shown). Rotor poles 58 andhubs 162 and 164 are a solid structure or a laminated structure heldtogether with interlocks, rivets, bolts, and/or other fasteners. Magnets56 are inserted into radial apertures 46 or a magnetic material isinjected therein to form magnets 56. A non-magnetic polymer is injectionmolded into space 166 between outer hub 164 and inner hub 162, betweenrotor outer edge 40 and magnet 56, and between magnet 56 and outer hub164. In this way, rotor poles 58 and magnets 56 are magneticallyisolated on the outer diameter of rotor 36 and rotor poles 58 andmagnets 56 are oriented at a predetermined diameter equal to a desiredrotor diameter. Magnets 56 may be magnetized before insertion into themold or may be magnetized after the molding process. Alternatively,rotor poles 58 and hubs 162 and 164 are located in the mold and ahigh-strength material with a high first processing temperature isinjection molded in space 166 to form a central hub. Permanent magnets56 are then inserted into radial apertures 46 defined between rotorpoles 58, and a lower second processing temperature material is moldedin the remaining space around magnets 56. This two-step molding processprevents performing the high processing temperature material injectionwhile magnets 56 are in the mold, which may reduce the effectiveness ofmagnets 56.

In the embodiments described above, electric motor 10 includes statorcore 28, which includes windings 32. Typically, windings 32 arefabricated from copper, which makes up a significant part of the motormaterial cost. In one exemplary embodiment, windings 32 are formed fromaluminum instead of copper. However, aluminum has approximately 60% ofthe resistivity of copper and lowers motor efficiency. One way to reducethe loss of efficiency is to increase the length of stator core 28 androtor core 36. However, increased motor length is undesirable from amaterial cost standpoint as well as an application standpoint (e.g. thelonger motor may not fit in a desired user application). In order toreduce or recover the efficiency loss associated with aluminum windingswhile minimizing an increase in the length of motor 10, radiallyembedded permanent magnet rotor 36 described above is used. Radiallyembedded permanent magnet rotor 36 generates more flux than other commonrotor designs using ferrite magnets. Radially embedded permanent magnetrotor 36 results in increased torque, which makes up for the loss inefficiency.

FIG. 20 illustrates another exemplary electric motor 200 that is similarto electric motor 10, except electric motor 200 includes a rotor 202having a different magnet arrangement than rotor core 36 above. Inelectric motor designs having rotor cores without radially embeddedpermanent magnets (e.g. surface mounted magnet rotors), motor efficiencyis also reduced when copper windings are replaced with aluminum windings32. In order to overcome efficiency loss, electric motor 200 includesone or more permanent magnet 204 and aluminum windings 206. In theexemplary embodiment, permanent magnet 204 is fabricated from a highergrade ferrite material that has remnance (Br) magnetic flux densityhigher than about 0.40 Tesla (about 4 K Gauss). Alternatively, permanentmagnet 204 is fabricated from any material that enables electric motor200 to increase flux density while using lower-cost materials.

Moreover, in the exemplary embodiment, windings 206 are designed oroptimized so that the knee of the speed-torque curve of motor 200 isoperating at substantially the full load operating point required bymotor 200 for a desired application. The knee of the speed-torque curveis the motor speed at which the motor transitions from the voltagecontrol region to the current control region. FIG. 21 illustrates anexemplary plot of speed and torque for motor 200. The desired full loadoperating point is represented by point 208. Line 210 plots thespeed-torque curve of an electric motor without optimized windings.Point 212 represents the knee of the speed-torque curve of line 210.Line 214 plots the speed-torque curve of electric motor 200 withoptimized windings 206. Point 216 represents the knee of thespeed-torque curve of line 214. In the exemplary embodiment, knee 216 ofthe curve is dropped closer to full load operating point 208 by addingmore turns in windings 206. By use of magnets 204 and/or optimizedwinding design of windings 206, electric motor 200 is configured torecover all or at least a portion of the efficiency losses due tofabricating windings 206 from aluminum, without increasing the length ofelectric motor 200.

FIG. 22 illustrates another exemplary electric motor 300 that is similarto electric motors 10 and 200, except electric motor 300 includes amodified rotor 302 and windings 306. In the exemplary embodiment,modified rotor 302 is a radially embedded permanent magnets rotor andreplaces an original rotor (not shown). Alternatively, modified rotor302 is a surface mounted permanent magnet rotor or an internal permanentmagnet rotor. Motor 300 is a motor platform that is designed foroperation at a typical design speed, which is the normal operating speedof a motor for a given rotor/stator construction. However, in theexemplary embodiment, motor 300 is desired to operate at a higher speed.Operation of motor 300 with the original rotor at the higher speedgenerates increased frequency, resulting in increased core loss anddecreased efficiency. In the exemplary embodiment, motor 300 is fittedwith modified rotor 302 to increase efficiency. Typically, rotor coresand stator cores have the same length. In the exemplary embodiment,rotor 302 is reduced in length compared to a stator core 28 so thatrotor 302 generates less flux at the higher speeds. This enables motor300 to operate at the higher desired operating speeds with increasedefficiency with reduced core loss. Moreover, shorter length rotor 302requires less material and reduces the cost of motor 300. Additionally,flux density output of motor 300 is reduced by reducing the number ofturns of windings 306. In the exemplary embodiment, windings 306 arefabricated from copper and/or aluminum. Alternatively, windings 306 arefabricated from any suitable material that enables motor 300 to functionas described herein. In another embodiment, flux density output of motor300 is reduced by using a lower remnance permanent magnet, a magnet witha reduced width, and/or a magnet with a shallower radial depth.

In the exemplary embodiment, the length of rotor 302 (compared to theoriginal rotor) is dependent on the higher operation speed desired bythe new application. For example, in one embodiment, motor 300 isoriginally designed to operate at a typical speed of 1,200 RPM andincludes a stator and rotor with lengths of approximately 1.75 inches.The new motor application requires motor 300 to operate at 6,000 RPM,and motor 300 is fitted with modified, reduced-length rotor 302, whichis approximately 1 inch in length. As described above, motor 300 ismodified with rotor 302 and/or windings 306 to enable motor 300 to beused for high speed applications without having to design a new motorplatform specifically designed for the high speed application.

Described herein are exemplary methods, systems and apparatus utilizinglower cost materials in a permanent magnet motor that reduces oreliminates the efficiency loss caused by the lower cost material.Furthermore, the exemplary methods system and apparatus achieveincreased efficiency while reducing or eliminating an increase of thelength of the motor. The methods, system and apparatus described hereinmay be used in any suitable application. However, they are particularlysuited for HVAC and pump applications.

Exemplary embodiments of the electric motor assembly are described abovein detail. The electric motor and its components are not limited to thespecific embodiments described herein, but rather, components of thesystems may be utilized independently and separately from othercomponents described herein. For example, the components may also beused in combination with other motor systems, methods, and apparatuses,and are not limited to practice with only the systems and apparatus asdescribed herein. Rather, the exemplary embodiments can be implementedand utilized in connection with many other applications.

Although specific features of various embodiments of the disclosure maybe shown in some drawings and not in others, this is for convenienceonly. In accordance with the principles of the disclosure, any featureof a drawing may be referenced and/or claimed in combination with anyfeature of any other drawing.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. A permanent magnet rotor for use with a stator,said rotor comprising: at least one permanent magnet; and asubstantially cylindrical rotor core comprising an outer edge, a hubhaving an inner edge defining a central opening, and a plurality ofindependent pole pieces, wherein each pole piece comprises a distinctstructure spaced from said hub radially about said central opening,wherein said rotor core further comprises at least one radial apertureextending radially from said outer edge through said rotor core betweenadjacent independent pole pieces, said at least one aperture configuredto receive said at least one permanent magnet, and wherein said rotorcore further comprises at least one shorting lamination coupled to saidrotor core, said at least one shorting lamination comprising a pluralityof connected pole pieces positioned radially about and coupled to acentral hub, wherein each of said connected pole pieces is coupled toone said independent pole piece.
 2. The rotor of claim 1, wherein saidrotor core further comprises a stack of laminations.
 3. The rotor ofclaim 1, wherein each said connected pole piece comprises a pole outeredge and a pole inner edge, wherein a pair of adjacent pole pieces areconnected at said pole inner edges.
 4. The rotor of claim 1, whereinsaid rotor core further comprises a first shorting lamination coupled toa first end of said rotor core and a second shorting lamination coupledto a second end of said rotor core.
 5. The rotor of claim 1, whereinsaid rotor core further comprises first and second half-poles, saidshorting lamination positioned between said first and second half-poles.6. The rotor of claim 1, wherein said at least one radial aperturecomprises opposed first and second edges, wherein a distance betweensaid first and second opposed edges is greater than three multiples ofan air gap distance between said rotor outer edge and an inner edge ofthe stator.
 7. The rotor of claim 1, wherein said at least one radialaperture comprises opposed first and second edges, wherein a distancebetween said first and second opposed edges is greater than fivemultiples of an air gap distance between said rotor outer edge and aninner edge of the stator.
 8. The rotor of claim 7, further comprising afirst protrusion extending from said first edge into said at least oneradial aperture, said first protrusion configured to facilitateretaining said at least one permanent magnet within said at least oneradial aperture.
 9. The rotor of claim 1, wherein said at least oneradial aperture comprises opposed first and second edges, wherein adistance between said first and second opposed edges is greater than tenmultiples of an air gap distance between said rotor outer edge and aninner edge of the stator.
 10. The rotor of claim 1, wherein a thicknessof said at least one shorting lamination is less than 12% of the totallength of said rotor core.
 11. The rotor of claim 1, wherein a thicknessof said at least one shorting lamination is less than 2% of the totallength of said rotor core.
 12. The rotor of claim 1, wherein a thicknessof said at least one shorting lamination is less than 1% of the totallength of said rotor core.
 13. An HVAC system comprising at least one ofa fan, a blower and a compressor motor, wherein said at least one fan,blower and compressor motor comprises a permanent magnet rotor asdescribed in claim
 1. 14. A fluid pumping system comprising a motor,wherein said motor comprises a permanent magnet rotor as described inclaim
 1. 15. A method of manufacturing a permanent magnet rotor for usewith a stator, the method comprising: providing a substantiallycylindrical rotor core comprising an outer edge, a hub having an inneredge defining a central opening, and a plurality of independent polepieces, wherein each pole piece includes a distinct structure spacedfrom the hub radially about the central opening; coupling at least oneshorting lamination to the rotor core, the shorting laminationcomprising a plurality of connected polepieces positioned radially aboutand coupled to a central hub, wherein each of the connected pole piecesis coupled to one independent pole piece; positioning the plurality ofindependent pole pieces to form at least one radial aperture extendingradially from the outer edge through the rotor core between adjacentindependent pole pieces; and inserting a permanent magnet into the atleast one radial aperture.
 16. The method of claim 15, wherein the rotorcore further comprises a stack of laminations.
 17. The method of claim15, wherein said positioning the plurality of independent pole piecescomprises positioning adjacent pole pieces from each other at a distancegreater than three multiples of an air gap distance between the rotorouter edge and an inner edge of the stator.
 18. The method of claim 17,further comprising forming a first protrusion extending from a firstedge of the at least one radial aperture, the first protrusionconfigured to facilitate retaining the at least one permanent magnetwithin the at least one radial aperture.
 19. The method of claim 15,wherein the thickness of the at least one shorting lamination is lessthan 12% of the total length of the rotor core.
 20. The method of claim15, wherein the thickness of the at least one shorting lamination isless than 2% of the total length of the rotor core.
 21. The method ofclaim 15, wherein the thickness of the at least one shorting laminationis less than 1% of the total length of the rotor core.
 22. The method ofclaim 15, wherein the at least one shorting lamination is coupled to anend of the rotor core.
 23. The method of claim 15, further comprisingproviding an HVAC system comprising at least one of a fan, a blower anda compressor motor, and coupling the permanent magnet rotor to the atleast one fan, blower and compressor.
 24. The method of claim 15,further comprising providing a fluid pumping system comprising a motor,and coupling the permanent magnet rotor to the motor.
 25. A permanentmagnet rotor for use with a stator, said rotor comprising: at least onepermanent magnet; and a substantially cylindrical rotor core comprising:an outer edge; a hub having an inner edge defining a central opening; aplurality of independent pole pieces, wherein each pole piece includes adistinct structure spaced from said hub radially about said centralopening; at least one radial aperture comprising opposed first andsecond edges, said radial aperture configured to receive said at leastone permanent magnet and extending radially from said outer edge throughsaid rotor core between adjacent independent pole pieces, wherein adistance between said first and second opposed edges is greater thanthree multiples of an air gap distance between said rotor outer edge andan inner edge of the stator; and at least one shorting laminationcoupled to said rotor core, wherein a thickness of said at least oneshorting lamination is less than 12% of the total length of said rotorcore.